When a fault occurs on a power transmission line between two stations, independently of whether it is an ac line or a high-voltage direct current line, that is, a so-called HVDC line, it is of great value to rapidly determine the distance from any or both of the stations to the fault location. For this purpose, so-called line fault locators, known under the generally accepted abbreviation LFL, are provided.
Known methods for such fault-distance determination deal predominantly with fault-distance determination on ac lines. A summary of and a short description of methods and measuring principles for this purpose are given, for example, in an article "Fault Location Methods for Overhead Lines", published in Trans AIEE, 7G (1957), part III, pp. 518-530. This article describes a measuring principle which comprises detection of a fault and evaluation of the distance to the fault as a travelling wave time principle. This measuring principle is also used in connection with the present invention.
When a fault occurs on a line, a transient occurs between the states prior to and after the fault in the form of travelling waves which propagate from the fault, in respective directions, towards the two stations. There are different ways of measuring the propagation times of the travelling waves in order to locate the fault. Since this invention relates to a method of measurement for these travelling waves, the actual evaluation method for the fault-distance determination will be dealt with very briefly.
A very common method, which is described in an article "Automatic Fault Locator for Bonneville Power Administration" by D. J. Marihart and N. W. Haagenson, published as Conference Paper, IEEE, PES, July 1972, is to determine the difference in time between the arrival of the travelling waves at the two stations. With knowledge of this difference in time, it is, in principle, relatively simple to obtain a fault-distance determination. The direct measurement of the difference in time may be performed in different ways, but normally takes place with the aid of synchronized clocks at the two stations. Variants of such determination are also described in U.S. Pat. No. 3,609,533, "Fault Location System for Determining Distance of Fault on a Transmission Line From a Predetermined Position Thereon". During fault-distance determination while determining the time at the two stations, it is sufficient to detect either the voltage or the current which is associated with the travelling waves.
To eliminate the communication between the two stations which is necessary according to the above-mentioned method, the fault-distance determination may also be performed by measuring the travelling wave, or waves reflected by the travelling wave, at only one of the stations. To be able to determine the fault distance, however, this method requires determination of both the current and the voltage which are associated with the travelling wave.
The references and methods mentioned above relate to fault-distance determination on ac lines, where, for measurement of the current and the voltage which are associated with these waves, current and voltage transformers are used.
Using travelling waves for fault determination on dc lines is also possible, and an article entitled "Development and Field-Data Evaluation of Single-End Fault Locator For Two Terminal HVDC Transmission Lines" by M. Nado, E. O. Schweitzer and R. A. Baker, published in IEEE, Vol. Pas-104, No. 12, December 1985, pp. 3524-3537, describes a method for fault-distance determination based on measurement at one station of both the current and the voltage which are associated with both travelling waves and reflected waves. The article shows that measurement of the current is performed via shunt resistances and that the voltage is measured with the aid of "compensated" voltage dividers.
The above-mentioned invention deals with determination of travelling waves on HVDC lines and relates to a method wherein the difference in time is determined between the point where travelling waves from a fault reach the end stations of the line. It is thus assumed that the time determination takes place with synchronous clocks and communication between the two stations. According to the above, this measuring and determination principle means that only the voltage or the current which is associated with the travelling wave is needed for the fault-distance determination.